1. Field of the Invention
The present invention relates generally to calibrating test equipment and, more particularly, to a method of maintaining calibration of scanning electron microscopes (SEMs) such that repeatable performance may be obtained.
2. Description of the Related Art
This section is intended to introduce the reader to various aspects of art which may be related to various aspects of the present invention which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
The scanning electron microscope (SEM) is used to create magnified images. As with all microscopes, the main purpose is magnification with clarity. Conventional light microscopes use multiple glass lenses to bend light waves to create a magnified image. The SEM is able to achieve much higher magnification than the light microscope largely because the SEM uses electrons instead of light waves to create the image. The SEM emits a beam of high energy electrons through a column in which a vacuum has been established. This beam travels downward and is focused into a very fine point by a series of magnetic lenses. The focused beam moves back and forth across the specimen, scanning it row by row. As the electron beam contacts the specimen at each spot, secondary electrons are displaced from its surface and electrons are scattered. These electrons are then counted by a detector, which sends signals to an amplifier. The number of electrons emitted from each spot on the sample is determined, and the data is combined to create the final image. The amount of scattering of electrons in any direction is a function of the angle made by the beam with the surface of the specimen and as a result, the image produced has a three dimensional appearance.
One use of SEMs is in the manufacture of semiconductor devices. SEMs are typically used in semiconductor manufacturing to measure the dimensions of articles of manufacture to ensure that certain specifications are met. This primarily involves measuring what is known as the critical dimension (CD) of an object such as an integrated circuit die or feature thereof. The CD is generally the length or width of a line, space, or contact. When the CD of a particular article is outside of a defined range, that article may not meet required specifications. If, for example, a wafer has been over exposed, an electrical contact may become enlarged such that it is beyond the specification for that part. This error can be made evident by utilizing an SEM to measure the CD. When a product is found to be defective for having a measurement that is out of specification, the product may be scrapped or subjected to corrective measures.
An SEM must be manually adjusted on a regular basis to keep it at optimum performance. These manual adjustments involve adjustment of resolution, magnification, and lens aberration. Because these adjustments are subjective, variations can be introduced resulting in distortion and incorrect measurement. For example, magnification in one part of the image may be different from another part of the image. The undesirable variation in image magnification may result in errors in categorizing devices measured with the SEM. In other words, devices measured with the SEM may be incorrectly categorized as out of specification. More problematic is the possibility that devices that in fact do not meet a given specification could be categorized as meeting the specification. Erroneous categorization of devices that are actually out of specification could result in subsequent problems such as failure of a device in the field or other performance problems.
A known method of calibration involves measuring a line and using the measurements to judge subsequent SEM setups as either acceptable or unacceptable. After the initial measurements of the line are obtained, the same line may be re-measured at periodic intervals (for example, daily or weekly). If measurements taken at later times are within a predetermined tolerance of the original measurements, the setup of the device may be deemed to be acceptable. If subsequent measurements are outside of a predetermined range, the machine may be subject to corrective action, such as modifying certain settings.
One problem with using lines as setup guides is that carbon deposits may accumulate on the line to be measured and actually change the length of the line. Thus, subsequent measurements of the line may not accurately reflect the true calibration of the SEM. SEMs are typically used to measure very small structures, so even small buildups may be problematic.
Another known practice for calibration involves what is known as a pitch test. In a pitch test, the pitch between a space and a line are measured. The pitch (or orientation) of the space relative to the line has been found not to change significantly even if deposits are present. Accordingly, the pitch test somewhat overcomes the problem of carbon deposits. However, the pitch test method does not take into account that astigmatism may have been introduced into the setup of the SEM. Corrections to account for astigmatism may be performed through a process known as “wobbling.” In wobbling, an operator may wobble the lens of the SEM to attempt to stabilize the image. Wobbling, however, is prone to subjective error.